Natural gas is very important both as fuel and as a basic industrial raw material. The composition of raw natural gas varies from field to field. It may contain more than 95% methane with small amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide, or water vapor. On the other hand, it may contain up to 15% ethane, propane, butane, or combinations of these components. For safety reasons, the Btu rating of natural gas to be carried through a pipeline is controlled within a narrow range (950-1,050 Btu/ft.sup.3). However, because of the higher Btu values of ethane, propane, butane, and pentane, natural gas streams containing significant amounts of these components are too high in Btu value to be fed directly to a pipeline or to be used directly as fuel. Equally important, higher hydrocarbons are too valuable industrially to be burned as secondary components in gas. Thus, the natural gas stream must undergo some form of treatment to remove undesirable components, to bring the Btu value to the standard level, and to recover the valuable higher hydrocarbons.
U.S. Pat. No. 4,857,078 discloses a process for removing propane, butane and higher hydrocarbons from natural gas using a composite membrane with an ultrathin rubbery selective layer.
The optimum separation membrane for use in any gas-separation application combines high selectivity with high flux. Thus the membrane-making industry has engaged in an ongoing quest for membranes with improved flux/selectivity performance.
Gas and vapor permeation through polymer membranes is usually rationalized by the solution-diffusion model. This model assumes that the gas phases on either side of the membrane are in thermodynamic equilibrium with their respective polymer interfaces, and that the interfacial sorption and desorption process is rapid compared with the rate of diffusion through the membrane. Thus the rate-limiting step is diffusion through the polymer membrane, which is governed by Fick's law of diffusion. In simple cases, Fick's law leads to the equation EQU Q=(J/.DELTA.p)=(D.multidot.S)/l, (1)
where Q is the pressure-normalized flux cm.sup.3 (STP)/cm.sup.2.s.cmHg!, J is the volumetric flux per membrane area cm.sup.3 (STP)/cm.sup.2.s!, D is the diffusion coefficient of the gas or vapor in the membrane cm.sup.2 /s! and is a measure of the gas mobility, l is the membrane thickness, S is the Henry's law sorption coefficient linking the concentration of the gas or vapor in the membrane material to the pressure in the adjacent gas cm.sup.3 (STP)/cm.sup.3.cmHg!, and .DELTA.p is the pressure difference across the membrane. The product D.multidot.S can also be expressed as the permeability coefficient, P, a measure of the rate at which a particular gas or vapor moves through a membrane of standard thickness (1 cm) under a standard pressure difference (1 cmHg). As can be seen from Equation 1, the pressure-normalized flux is inversely proportional to the membrane thickness.
For a given membrane material, the ideal selectivity, .alpha..sub.A,B, for gas A over B is defined as the ratio of the permeability coefficients of the gases: EQU .alpha..sub.A,B =P.sub.A /P.sub.B =(D.sub.A /D.sub.B).multidot.(S.sub.A /S.sub.B), (2)
where P.sub.A and P.sub.B are the permeability coefficients of gases A and B, respectively, as determined from the measured pressure-normalized fluxes of two gases, the fluxes being measured separately, each with a pure gas sample, through a defect-free membrane sample of the same thickness, and being expressed in cm.sup.3 (STP)/cm.sup.2.s.cmHg or other consistent units. Selectivity, as defined in Equation 2, is a product of two terms. The first term is the ratio of the diffusion coefficients and is usually called the mobility selectivity. This term reflects the relative size of the permeants. In the case of the separation of organic compounds from permanent gases, such as nitrogen, the diffusion coefficient of the organic vapor is always less than that of nitrogen, so the mobility selectivity term is less than one. The second term is the sorption selectivity and reflects the relative sorption of the two permeants. In general, the more condensable the component, the higher its sorption. Thus, in the case of the separation of a more condensable organic compound from a permanent gas or less condensable organic or inorganic compound, the sorption selectivity term will usually be greater than one. Whether a particular membrane material is selective for the more condensable components of a gas mixture depends on the balance of these two terms for that material.
Diffusion coefficients are generally several orders of magnitude higher in rubbery polymers than in glassy polymers and are substantially less dependent on the penetrant size, particularly in the case of large, condensable molecules. As a result, the selectivity of rubbery polymers is mainly determined by the sorption term and rubbery materials are usually condensable-selective. Glassy polymer selectivities, on the other hand, are dominated by the diffusion term and glassy polymers are usually gas-selective. Data illustrating the standard behavior of rubbery and glassy polymers are shown in FIG. 2, originally prepared by the German company, GKSS. Only rubbery polymers, therefore, have been considered useful for separating condensable organic compounds from other gases and vapors.
In recent years, some polymer materials with unusually high permeabiliites have been synthesized. Perhaps the best known of these, and representative of the class, is polytrimethylsilylpropyne (PTMSP), a polymer synthesized by T. Masuda et al. in Japan. Although PTMSP is glassy, up to at least about 200.degree. C., it exhibits an oxygen permeability of 10,000 Barrer or above, more than 15 times higher than that of silicone rubber, previously the most permeable polymer known. The selectivity for oxygen/nitrogen, however, is low (1.5-1.8). The high permeability appears to be associated with an unusually high free-volume within the polymer material, and has been confirmed with many examples of pure gases and vapors, including oxygen, nitrogen, hydrogen, helium, methane, ethane, propane, butane and higher hydrocarbons, sulfur hexafluoride and carbon dioxide. These pure-gas data suggest that PTMSP will exhibit poor selectivity for most gas separations. For example, a paper by N. A. Plate/ et al. ("Gas and vapor permeation and sorption in poly(trimethylsilylpropyne", Journal of Membrane Science, Vol. 60, pages 13-24, 1991) gives polymer permeabilities of 2.6.times.10.sup.-7 cm.sup.3 (STP).cm/cm.sup.2.s.cmHg for oxygen and 1.5.times.10.sup.-7 cm.sup.3 (STP).cm/cm.sup.2.s.cmHg for nitrogen, giving a calculated selectivity of 1.7. The same reference gives polymer permeabilities of 2.7.times.10.sup.-7 cm.sup.3 (STP).cm/cm.sup.2.s.cmHg for methane, 1.9.times.10.sup.-7 cm.sup.3 (STP).cm/cm.sup.2.s.cmHg for propane and 2.3.times.10.sup.-7 cm.sup.3 (STP).cm/cm.sup.2.s.cmHg for n-butane, giving a calculated selectivity for propane/methane of 0.7 and for butane/methane of 0.85. A paper by M. Langsam et al. ("Substituted Propyne Polymers. I. Chemical surface modification of poly 1-(trimethylsilyl)propyne! for gas separation membranes", Gas Separation and Purification, Vol. 2, pages 162-170, 1988) gives a carbon dioxide/methane selectivity for PTMSP of 2.07, compared with 9.56 for silicone rubber. A paper by K. Takada et al. ("Gas Permeability of Polyacetylenes Carrying Substituents", Journal of Applied Polymer Science, Vol. 30, pages 1605-1616, 1985) includes the statement that: "Very interestingly, poly 1-(trimethylsilyl)-1-propyne! films show permeability coefficients as high as 10.sup.-7 -10.sup.-6 to every gas. However, permselectivities of these films for two different gases are relatively poor." Thus the material was characterized, at least initially, as of great academic interest, because of its extraordinary permeability, but exhibiting selectivities too low for commercial use.
As soon as its remarkable permeability properties were announced, PTMSP attracted attention from the membrane community at large. A number of experimenters reported that the permeation properties of PTMSP appear to be unstable over time, raising further doubts as to the usefulness of the material for practical applications. In particular, the oxygen permeability was found to drop dramatically. For example, Masuda et al. found that the oxygen permeability fell to about 1% of its orginal value when the membrane was left at room temperature for several months.
More recently, the consensus of opinion in the art has been that the loss in permeability arises primarily from sorption of volatile materials from the environment of the membrane. If the membrane is mounted in a system containing a vacuum pump, for example, vaporized or aerosol vacuum oil may be sorbed into the membrane material. A similar effect may occur if the membrane is simply standing in the air for prolonged periods. For example, a paper by T. Nakagawa et al. ("Polyacetylene derivatives as membranes for gas separation", Gas Separation and Purification, Vol. 2, pages 3-8, 1988) states that "the PMSP membrane showed strong affinity to volatile materials. It was considered that, in addition to the thermal hysteresis, the reason for unstable gas permeability is the adsorption of volatile materials existing in the atmosphere."
This property has been turned to advantage by several workers. For example, the above-cited Nakagawa paper also discusses the performance of PTMSP membranes that have been deliberately exposed to a variety of additives, including dioctyl phthlate (DOP) and polyethylene glycol (PEG). The treated membranes exhibited permeation properties stable over time, and, although the oxygen permeability was reduced from 8,000 Barrer to about 300-400 Barrer, the oxygen/nitrogen selectivity improved from 1.6 to 3.3, rendering the membranes "prospective as membranes for oxygen enrichment". Similar results have been reported by S. Asakawa et al. ("Composite membrane of poly 1-(trimethylsilyl)-propyne! as a potential oxygen separation membrane", Gas Separation and Purification, Vol. 3, pages 117-122, 1989), who apparently produced stable PTMSP membranes by coating the PTMSP layer with a protective layer of silicone rubber, and who also concluded that, "This membrane, therefore, may be promising for industrial oxygen separation." M. Langsam et al. (U.S. Pat. No. 4,859,215, Aug. 22, 1989, assigned to Air Products and Chemicals, Inc.) added Nujol oil, silicone oil or ethylene oxide-based surfactants to the casting solution when preparing PTMSP membranes. The membranes showed permeation properties stable over time, reduced permeabilities and improved selectivities for oxygen/nitrogen and carbon dioxide/nitrogen.
Other attempts to modify the material to increase its selectivity have been made. For example, U.S. Pat. No. 4,657,564, to M. Langsam, assigned to Air Products and Chemicals, Inc., describes a surface fluorination technique that increases the oxygen/nitrogen selectivity by at least 50% over its unmodified value. Thus, use of the material has focued on oxygen/nitrogen separation, and ways in which the extraordinary oxygen permeability can be preserved yet the low oxygen/nitrogen selectivity enhanced.
Almost all of the permeation data that have been published concern pure gas experiments. However, a study by S. R. Auvil et al. ("Mechanisms of gas transport in poly(1-trimethylsilyl-1 propyne"), Polymer Preprints, Vol. 32(3), pages 380-383, 1991) was carried out using mixtures of a heavy gas (carbon dioxide or sulfur hexafluoride) and a light gas (helium or nitrogen). The study showed that the permeability of the light gas is substantially reduced in the presence of the heavy gas. It was postulated that the heavy gas is adsorbed onto surfaces of voids within the structure of the polymer and may be transported through the material by surface diffusion, and further that these surface layers may build up and block diffusion of the light gas through the void areas. The net result was an increase in the selectivity for the heavy gas over the light gas when measured with gas mixtures rather than calculated from pure gas permeabilities.
To summarize the above discussion, it may be seen that glassy, high-free-volume polymers, of which PTMSP is the most widely studied example, exhibit unusual gas transport properties. These properties do not conform to, and do not appear to follow from the standard solution/diffusion model of gas transport. Furthermore, the properties are affected in a not fully understood fashion by sorption of a variety of volatile materials. Behavior with mixed gases has not been studied, except in a very limited way, but the results obtained again are inconsistent with those obtained from conventional polymer materials.